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1. Field of the Invention
This invention relates generally to electrical and electronic systems, and, more particularly, to electrical and electronic interface hardware for establishing high speed connections between different portions of an automatic test system.
2. Description of Related Art
A need commonly arises in automatic test equipment, or xe2x80x9ctesters,xe2x80x9d to convey electrical signals from one location to another without significant loss of signal integrity. High-quality signal transmission is particularly critical at a tester""s device interface, where electrical signals are communicated between the tester and a device under test, or xe2x80x9cDUTxe2x80x9d.
Probe towers are commonly used for communicating electrical signals at device interfaces. As is known, xe2x80x9cprobe towersxe2x80x9d are arrays of double-ended, spring-loaded pins, which extend tester signals vertically from one plane to another. Probe towers may be used to communicate signals between a tester and a peripheral, such as a prober or handler, or between different planes within a tester or peripheral.
Conventional probe towers include arrays of unshielded, double-ended spring probes, which are identical for both signals and grounds. Although these probe towers perform adequately for slower signals (e.g., audio range), they do not tightly control characteristic impedance, stray capacitance, or stray inductance, and are thus ill suited for higher speed applications.
Newer designs improve performance by replacing the unshielded spring probes with coaxial pins. An example of a coaxial probe tower can be found in U.S. Pat. No. 6,037,787, entitled, xe2x80x9cHigh Performance Probe Interface for Automatic Test Equipment,xe2x80x9d which is incorporated herein by reference in its entirety. That patent discloses a probe tower in which coaxial pins have a central spring probe concentrically held within a conductive tube by an insulative material. A separate spring probe is electrically connected to the outside of the conductive tube for conveying a ground connection. The spacing between the central spring probe and the conductive tube maintains a well-controlled and substantially constant characteristic impedance over the height of the probe tower. Stray capacitance is virtually eliminated, and stray inductance is significantly reduced.
Recently, commercial products have come to market that include SerDes (Serializer/Deserializer) and SONET (Synchronous Optical Network) devices, which produce signals in excess of 1 GHz. Because the effects of inductance increase with frequency, even the small stray inductance of the coaxial probe tower can significantly impair signal integrity above 1 GHz. In addition, SerDes and SONET devices communicate their signals differentially, i.e., their signals are conveyed as pairs that vary with respect to each other, rather than with respect to ground. For accurately testing these devices using automatic test equipment, a new type of probe tower is needed that preserves signal integrity for high speed signals, particularly for signals that are conveyed differentially.
With the foregoing background in mind, it is an object of the invention for a probe tower to preserve signal integrity for differential signals operating at high speeds.
To achieve the foregoing object, as well as other objectives and advantages, a differential probe assembly for an automatic test system includes first and second coaxial portions and a ground portion. Each coaxial portion includes an outer conductor with first and second ends and at least one insulator positioned therein for holding a center conductor. First and second contact pins extend from the center conductor at the first and second ends, respectively. The ground portion also has first and second ends from which first and second ground pins respectively extend. The outer conductors of the first and second coaxial portions are electrically connected to the ground portion, for establishing a ground connection.